I had heard that the Terminal where the Scotia Prince would dock was contaminated with asbestos (that was some time ago). I don't know if they have cleaned it up yet, or it is still contaminated.
I believe that the Scotia Prince departs at night and arrives in Yarmouth in the morning, then departs again from Yarmouth in the morning and arrives in Portland at night.
Going, I didn't mind because we had a cabin (make sure you upgrade with a bath with a shower, or you'll share a bath down the hall), but coming back I felt like I was waisting a day with nothing to do (we don't gamble).
Next time I think that we'll come back via New Brunswick, at least I'll be riding and seeing stuff other than open water.
Another alternative is the "Cat" out of Bar Harbor, Maine. I think the crossing is done in 3 hours versus 8-9 hours on the Scotia Prince, but you'll have to ride that much further north to pick it up. Your choice.
I think that this time of the year is a bit too cold up there. I'd suggest the last week in July and the first week in August. That's when we went.
Good luck and nothing but green lights.
Mike